The present invention relates to semiconductor devices. In particular, the present invention relates to nonvolatile memory devices.
Nonvolatile memory devices, electrically storing data and retaining the stored data until erasing signals are input, are required to be operable at low voltages for writing and erasing, and to have superior data retention. Generally, in cell regions of nonvolatile memory devices having multi-layer charge-storage layers, there are memory cells having multi-layer charge storage layers formed on substrates, and gate electrodes formed on the charge storage layers.
In the nonvolatile memory devices having the multi-layer insulation layers for charge storage, electrons or holes are discharged into the substrates from the charge-storage insulation layers, or injected into the charge-storage insulation layers from the substrates. Data states are read by evaluating variations of threshold voltages in accordance with potentials of the charge-storage insulation layers. In order to improve the efficiencies of writing/erasing data in the nonvolatile memory devices having the multi-layer insulation layers for charge storage, various methods have been studied. One such method involves changing materials of the charge-storage insulation layers.
Like a typical memory device, the nonvolatile memory device has a peripheral circuit region for operating the memory device in compliance with programmed information, as well as a cell region for containing data. In the peripheral circuit region, active elements such as transistors and passive elements such as resistors are arranged according to various structures. In order to drive the nonvolatile memory device by a low power-supply voltage, the transistors of the peripheral circuit region are required to be operated according to low threshold voltages and exhibit large saturation currents without increasing off-currents.